Silicon steel sheets or magnetic steel sheets are used as materials for magnetic cores of motors, generators, or transformers and during such use are required to exhibit small loss and large magnetic flux density.
Conventionally, magnetic steel sheets are classified into non-oriented magnetic steel sheets and oriented silicon steel sheets. Usually, in order to reduce core loss through suppression of occurrence of eddy current, magnetic steel sheets are arranged in layers into a laminated structure for use as magnetic cores of electric machinery. In this case, magnetization is effected in parallel with a sheet surface. Non-oriented magnetic steel sheets, when magnetized in parallel with a sheet surface, exhibit good magnetic characteristics in every direction and thus are favorably used in small-sized motors and the like. By contrast, oriented silicon steel sheets, when magnetized in a specific direction parallel to a sheet surface, i.e. in a direction parallel to their rolling direction, exhibit particularly excellent magnetic characteristics, but, when magnetized in other directions, have magnetic characteristics inferior to those of non-oriented magnetic steel sheets. Accordingly, oriented magnetic steel sheets are used in the form of combined laminated cores or wound cores, so that the rolling direction always corresponds to the direction of magnetization, thus enabling manufacture of transformers having a smaller loss.
An iron crystal has magnetic anisotropy. When a single crystal of iron is modeled as a cube, excellent magnetic characteristics are exhibited when magnetization is effected in a direction perpendicular to a face of the cube, i.e. in the direction of the &lt;001&gt; axis. In an oriented silicon steel sheet, the &lt;001&gt; axes of most iron crystal grains are parallel to a rolling direction, and the {110} planes are parallel to a sheet surface. This {110}&lt;001&gt; orientation is usually called Goss-orientation. A non-oriented magnetic steel sheet is manufactured under manufacturing conditions substantially similar to those for manufacture of an ordinary cold-rolled steel sheet, whereas an oriented silicon steel sheet is manufactured by the steps of cold-rolling steel containing Si in an amount of about 3%, subjecting the cold-rolled steel sheet to ordinary recrystallization annealing, and further annealing the recrystallized steel sheet at high temperature. During the high-temperature annealing, there must be carried out the so-called secondary recrystallization, in which Goss-oriented crystal grains are selectively grown through aid of sulfides and nitrides called inhibitors.
An oriented silicon steel sheet shows excellent magnetic characteristics in a rolling direction, but shows poor magnetic characteristics in other directions since the &lt;001&gt; axes of iron crystal grains constituting the steel sheet hardly exist in other directions. Accordingly, in an application such that magnetization is concurrently effected in a direction parallel to a rolling direction, and in a direction perpendicular to a rolling direction, as in the case of EI cores, a sufficient effect is not produced.
In contrast, if there is a steel sheet having a crystalline structure in which the &lt;001&gt; axes are parallel to a rolling direction, and the {100} planes are parallel to a sheet surface, the steel sheet exhibits excellent magnetic characteristics in a direction parallel to a rolling direction, and in a direction perpendicular to the rolling direction. In order to obtain a highly efficient small-sized transformer, such a steel sheet may not be formed into a wound core, but may be formed into an ordinary laminated core, such as an EI core or L core. Such a magnetic steel sheet having the {100}&lt;001&gt; orientation is called a doubly oriented magnetic steel sheet. Various methods for manufacturing a doubly oriented magnetic steel sheet have been studied, but a method has not been developed for manufacturing a doubly oriented magnetic steel sheet having satisfactory magnetic characteristics.
There is a known method for manufacturing a doubly oriented magnetic steel sheet, studied in the 1950s, in which a silicon steel sheet, having a thickness not greater than 0.3 mm, is annealed at a high temperature of 1200.degree. C. in a highly pure inert gas. In this method, during the process of high-temperature annealing, secondary recrystallization is effected through use of surface energy as a driving force, so as to grow {100}&lt;001&gt;-oriented crystal grains, thereby obtaining the crystalline structure of a doubly oriented magnetic steel sheet. However, the crystalline structure of a steel sheet manufactured by this method is coarse, and crystal grains have as large a size, as near 100 times the thickness of the steel sheet. The steel sheet fails to provide satisfactory magnetic characteristics and involves a problem of a large core loss when applied to a magnetic core.
Recently, there has been developed a magnetic steel sheet, having a crystalline structure, which is composed of relatively fine columnar crystal grains and in which the {100} planes are parallel to the surface of the steel sheet, as disclosed, for example, in Japanese Patent Application Laid-Open (kokai) No. 1-108345, etc.
According to the manufacturing method disclosed in Japanese Patent Application Laid-Open (kokai) No. 1-108345, a steel sheet containing C, Mn, and Si in appropriate amounts, and having a predetermined thickness, is first heated in a vacuum or in a weak decarburizing atmosphere so as to be gradually decarburized. In this case, the decarburization temperature range is such that steel in an austenite (.gamma.) region, or a two-phase region of austenite and ferrite (.gamma.+.alpha.), assumes complete ferrite (.alpha.) phase through decarburization down to a very low carbon concentration, sufficiently below 0.01%. Through gradual decarburization at a temperature in such a range, crystals having the &lt;001&gt; axis perpendicular to a sheet surface, or the {100} plane parallel to a sheet surface, are generated in a surface layer at high density. Subsequently, the steel sheet undergoes secondary decarburization annealing in a strong decarburizing atmosphere, in such a temperature range, that core steel is at the A.sub.1 point or higher, and is not higher than the temperature of the above primary decarburization annealing, so as to grow .alpha. grains from a sheet surface and sufficiently decarburize the entire steel sheet. As a result, there is obtained a magnetic steel sheet having numerous crystals whose {100} planes are parallel to a sheet surface.
In a surface layer, crystals having {100} planes parallel to a sheet surface grow well, particularly under gradual decarburization, for the following reason. Since the surface energy of the {100} plane of a ferritic grain is lower than that of a plane of another orientation, the ferritic grains grow preferentially. Also, the thinner the layer of the a phase, the greater the difference in the surface energy. The thus-formed ferritic grains in the surface layer serve as nuclei and grow into the interior of the steel sheet, while transformation is effected by decarburization and progresses from the .gamma. phase to the .alpha. phase.
According to another manufacturing method, disclosed in Japanese Patent Application Laid-Open (kokai) No. 1-252727, a steel sheet which undergoes final annealing in the above-mentioned method, is formed through a plurality of rolling steps, with intermediate annealing performed therebetween, to thereby obtain a silicon steel sheet having the {100}&lt;001&gt; crystallographic texture and an average grain size not greater than 1 mm. However, a crystalline structure obtained by this method is such that columnar crystal grains growing from both surfaces of a steel sheet toward the interior of the steel sheet collide at the central portion of the steel sheet, thereby becoming a fine structure whose grain size is about half the sheet thickness or smaller. In order to prevent the formation of a fine structure, annealing time may be extended so as to further grow crystal grains. However, the extension of annealing time causes the crystalline structure to become a duplex grain structure. The formation of a duplex grain structure causes a decrease in the strength of the {100}&lt;001&gt; crystallographic texture, and an impairment in magnetic characteristics represented by core loss.
According to still another manufacturing method, disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-173542, a tight coil of a steel sheet, with an oxide-based annealing separator held between spirals, or a lamination, composed of the oxide-based annealing separator and steel sheets arranged in alternating layers, is subjected to decarburization annealing under reduced pressure, to thereby grow in sheet surfaces a crystallographic texture having {100} planes parallel to the sheet surfaces through a single execution of annealing. Further, according to the publication, through selection of an adequate annealing separator, the removal of manganese can be effected during decarburization annealing, and this removal of manganese can accelerate the development of {100} plane orientation. However, in a magnetic steel sheet manufactured by this method, {100} planes are parallel to a sheet surface, but the &lt;001&gt; axes in a sheet surface are oriented differently from cubic orientation; thus the magnetic steel sheet has a {100}&lt;052&gt; type crystallographic texture. Accordingly, the method disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-173542, cannot be said to be that for developing the {100}&lt;001&gt; crystallographic texture.
As mentioned above, there are proposed several methods for manufacturing a magnetic steel sheet in which {100} planes are parallel to a sheet surface. However, in magnetic steel sheets manufactured by these methods, the orientation of the &lt;001&gt; axes in a sheet surface is different from that of {100}&lt;001&gt;, and even when the {100}&lt;001&gt; crystallographic texture is formed, magnetic characteristics are unsatisfactory. Accordingly, oriented magnetic steel sheets manufactured by these methods involve a problem of failure to exhibit satisfactory characteristics.